1. Field of the Invention
The present invention relates to a scanning system for an information displaying recorder, for example a printer, which utilizes a light beam, for example a laser light beam. More particularly, the present invention is a circuit for the control of a self-resonant galvanometer to which is affixed a mirror for scanning a light, or laser, beam in order that both the amplitude and the centering of a such a scanned light, or laser, beam deflected by such self-resonant galvanometer may be fixed and maintained most precisely.
2. Description of the Prior Art
An essential part of any information displaying recorder which utilizes a swept light beam, or non-impact printer, is the scanning system. The scanning system sweeps the light beam, nominally a laser beam such as is derived from a helium-neon laser or a laser diode, across a photo conductive target, nominally either a photo conductive drum or photo conductive paper. Diverse ways of scanning a light beam exist. These include a polygonal mirror and motor, a holographic disc and motor, a linear galvanometer and mirror, and a self-resonant galvanometer scanner. A survey of these diverse methods is contained in the article "Laser Scanning and Recording: Developments and Trends" occurring in LASER FOCUS/ELECTRO-OPTICS for February 1985 at pages 88-96.
In all cases of scanning with a light, or laser, beam in order to print or display information, data is used to turn the laser beam on and off in accordance with the presumed position of the scanning beam. In order to obtain the best possible appearance of the characters, it is essential that the presumed beam position and the actual beam position should agree as closely as possible. Otherwise, irregularities in the characters will occur, such as lines intended to be vertical appearing jagged. For motor-driven scanners, such as those employing a polygonal mirror or a holographic disc, one of the difficulties encountered is that the motors may speed up or slow down slightly, causing irregularities in the information displayed or printed.
For self-resonant galvanometer scanners, the control of which is the subject of the present invention, the amplitude of the oscillation and/or the center of the scan position may "hunt" up and down. This causes irregularities in pixel location resulting from error in the presumed position of the mirror of such self-resonant galvanometer scanner, and the resultant deflection of the light beam which would occur from such presumed mirror position. This imperfect repeatability, or "hunting", of a self-resonant galvanometer scanner results from the minute differentials of force operating on its delicate suspension system; a combination of magnetomechanical and electronic driver system imbalances.
Accompanying the diverse types of light deflecting, or laser beam, scanning systems within information displaying recorders, or laser printers, is existing technology directed toward minimizing errors. For example, in motor-driven scanners, precision or even air bearings are used to minimize velocity variation. However, as some errors still occur, a second laser beam from that first laser beam used for image projection, or printing, purposes is sometimes used. This second laser beam is projected onto a grating in order to generate a set of clock signals which, when detected, indicate the rate of travel of the mirror system.
More pertinent to the present invention are the prior art means for the control of the amplitude and the centering of the scanning mirror, and resultant scanned light beam, of the self-resonant galvanometer system. A general discussion of the prior art for control of self-resonant scanners is contained in the article, "Linearizing Resonant Scanners", appearing in the magazine LASERS AND APPLICATIONS for August 1985 at pages 65-69. Although primarily concerned with the problem of synchronizing a sinusoidal scanner velocity to external drive electronics, and the generation of fixed clocks such as will strobe data onto a laser beam being sinusoidally scanned, mention is made in this article of the severe problems of repeatability in beam scanning wherein phase delay and associated drift rates cause registry errors in the printed information.
The most straight-forward, and simplest, implementation of a self-resonant scanner linearization scene is an analog clock. However, the analog clocking of fixed data onto a sinusoidally variant light beam resultant from a self-resonant scanner suffers from poor performance characteristics stemming from thermal drift, resolution limits, and maximum pixel and scanning frequencies. A teaching of this method is contained in U.S. Pat. No. 3,978,281 to G. J. Burrer. An alternative linearization technique for self-resonant galvanometer scanners is to employ a grating technique like to that employed for rotating polygon scanners. An example of this scheme is taught in U.S. Pat. No. 4,212,018 to Ohnishi et al. Two methods of linearization are also discussed in the "Linearizing Resonant Scanners" article. These methods use beam positional sensors called Start-of-Scan (SOS) and End-of-Scan (EOS) sensors, which sensors, like as to sensors within the present invention, will sense the position of the scanned light beam. Resultant to this sensing, there is a feedback system implemented for control of the self-resonant galvanometer scanner. In particular, the scanner amplitude will be determined by comparing a previously stored value with the number of master clock cycles which have occurred between consecutive passages of the oscillatory light beam over the SOS sensor. The result of this comparison, performed in a microprocessor, is converted into a voltage by a digital-to-analog converter, with the output of such converter driving a sample and hold circuit. The value stored within such sample and hold circuit represents the amplitude error of the scanner, and is summed with the integral of the amplitude error and used to drive the automatic gain control input of an analog controller powering the self-resonant galvanometer scanner. In this manner, the amplitude of the scanner is attempted to be held to a variation which is a fraction of a pixel interval, and the operating angle is attempted to be held constant because the master oscillator will be phase- and frequency-locked to the scanner.
As a second digital variant of this linearization scheme, the count can be formed as the difference between an initial count occurring upon the passage of the beam over the SOS detector (representing the beginning of the image area) and the final count occurring at the time of passage of the beam over the EOS detector. The micro-processor will again use the time of occurrence information to generate corrections to scanner drift as occurred during the previous line-scan interval. The corrections are applied to set latches, which resultantly are interpreted by a digital-to-analog converter to provide fine tuning adjustment to the Voltage Controlled Output (VCO) master oscillator. The amplified output of such master oscillator is used to energize the drive of the self-resonant galvanometer.
Both prior art digital methods allow for correction of amplitude and frequency drifts in the self-resonant galvanometer scanner, eliminating the need for sophisticated analog phase-locked control units. But a limitation is that the fastest element in the hardware runs at the pixel clock rate. This pixel clock runs at very high frequencies, often upwards of 150 MHz. For example, an application calling for 20,000 pixels placed with an accuracy of 1 part in 200,000 (tolerable absolute non-cumulative pixel-placement error of 10 percent or less of the pixel-to-pixel spacing is nominal) using 80 percent of the scan angle with a scanner running at 200 Hz needs a master clock frequency of approximately 136 MHz. Such high pixel clocking rates are difficult to obtain with transistor-transistor logic, which is most reliable in the range below 50 MHz. An additional problem with the prior art schemes is that the stability and repeatability required of the analog voltage control oscillator and the associated driving digital/analog converters is very great. As previously stated, the minute differentials of force, which may occur from single scan to single scan, operating on the delicate suspension system of a self-resonant galvanometer scanner, combined with electronic driver system imbalances, produces repeatability inaccuracies in even the digital prior art methods for control of a self-resonant galvanometer scanner.
Still a further prior art scheme of clocking data to a laser diode, or other light modulation means, in a printer which uses a self-resonant galvanometer mirror is taught in U.S. Pat. No. 4,541,061 for Data Clocking Circuitry to the selfsame inventor of the present invention. A Start of Scan (SOS) signal from a photodiode signals when an exact position short of the leading edge of the image area (the paper) is reached. This signal resets a counter and enables a Voltage Controlled Oscillator (VCO) to start oscillating. This output goes to a counter which, in turn, supplies the data address for a memory. This memory contains the data which is converted to analog to control the speed of the VCO. This method supports speed corrections over the duration of the scan, but does not deal with establishing precise repeatability from scan to scan as is dealt with by the present invention.